Death rates from several kinds of cancer in people over 54 years old have increased dramatically over the last two decades in the United States and five other industrialized countries, according to a new study.

The figures suggest that a major shift may be underway in the relative importance of various kinds of cancer as causes of death in the developed world.

While lung cancer death rates show signs of starting to fall, death rates from brain tumors, multiple myeloma (a tumor of white blood cells), breast cancer and malignant melanoma (a skin cancer) have increased steeply in most of the six countries studied.

The study, which appears in Saturday's issue of the British medical journal The Lancet, said the observed changes in cancer death rates are "so great and rapid that it would be imprudent not to investigate their causes aggressively."

The causes of the change are not known but researchers suspect environmental toxins may play a role, along with improved diagnosis types and cases of cancer that would have been overlooked in the past.

The study found that the greatest overall increase in cancer death rates has occurred in people between the ages of 75 and 84, especially in men. Excluding mortality from lung cancer and stomach cancer, two environmentally caused tumors that appear to be declining, cancer death rates increased by at least 15 percent between 1968 and 1987 in that age group in all six countries: the United States, England and Wales, France, West Germany, Italy and Japan.

For some cancers in some countries, death rates have risen severalfold, said Devra Lee Davis, a professor of environmental and occupational medicine at Mt. Sinai School of Medicine and the report's principal author.

For instance, death rates from brain and nervous system tumors tripled among Americans between the ages of 75 and 84 during the period studied, she said. "Even though we know that there are good biological reasons to expect an increasing rate of cancer with age, why are older people getting more cancer {now} than they got in 1968?" she said.

Davis said it is too soon to tell whether the rising death rates are part of a continuing trend or whether they affect only the generation of people who are currently in their 50s, 60s and 70s.

Davis said the trend may not continue among today's younger people as they reach the age of the people in the study. For one thing, Davis noted that people now over 55 had less access to fresh fruit and vegetables throughout most of their lives than Americans do now. Many fruits and vegetables contain natural substances thought to protect against cancer.

"This generation smoked cigarettes containing more tar than today, and more of them smoked," she said. "They worked in dirtier workplaces. They had higher doses of diagnostic X-rays. More people worked in blue-collar jobs. They were on the cutting edge of using a lot of new chemicals in their households."

Among the study's specific findings:

Death rates from brain tumors more than doubled in all six countries among people between 75 and 84, and almost doubled in those 65 to 74.

Mortality from multiple myeloma increased by at least 80 percent in all six countries among people aged 75 to 84, and by 30 percent in those 65 to 74.

Death rates from breast cancer in women over 65 showed consistent increases, ranging from 10 percent to 60 percent depending on the country.

Death rates from malignant melanoma showed "rapid" increases among people over 45, Davis said.

Death rates from non-Hodgkin's lymphoma, another tumor of white blood cells, are also increasing in older people, although a small part of the increase may be due to AIDS.

Davis and her co-authors, scientists at the National Institute of Environmental Health Science, the World Health Organization and the British Office of Population Censuses, compared age-specific cancer mortality rates reported by the six countries in the study.

"We were quite surprised to see these trends emerging in all these countries," she said.